Traditionally, telephone switching systems rely upon sharing of control and switching facilities to achieve economy of cost. Such sharing is found in both central office machines, and in on-premises PBX's and intercom systems. The advent of low cost, relatively high speed functional processors has permitted the construction of relatively low cost on-premises switching machines with extensive lists of features for a business environment, and a number of manufacturers provide on premises line switching systems which serve from two to twelve central office lines and from eight to thirty two stations. Such systems generally rely upon the use of relatively high cost, proprietary station sets to achieve many of the system features along with two or more wire pairs to each station. The "get started cost" of the processing unit of such systems; the costs of required line cards and subscriber cards; and the high costs of the proprietary station sets and multi-pair station wiring are too high for application in a small business or in a residence setting.
A private branch exchange (PBX) and an end telephone office serving the PBX are interconnected by one or more trunk circuits. The trunk circuit or trunk circuits are identified in the national telephone numbering plan by a ten digit code. The PBX extensions are not identified in the national numbering plan; however, the PBX extensions are identified by a numbering plan which is local to the PBX. Incoming calls from a central office to a PBX are directed to an attendant position. The attendant answers an incoming call and completes the call to an extension as requested by the calling party. The calling party may identify the desired extension by its PBX extension number or the calling party may only identify an area of interest e.g., the service department and the attendant directs the call to the appropriate extension.
With the advent of DTMF calling from subscriber stations, numerous features have been added to both central office and on-premises intercom and PBX telephone switching systems. One such feature which has gained favor is termed "automated attendant service". This service, which is generally provided as an adjunct to an on-premises private branch exchange (PBX), is intended to relieve the duties of an attendant in serving calls which are incoming to the PBX.
In a dial PBX, extension to extension calls can be completed without the services of an attendant; however, prior to the availability of "automated attendant services", incoming calls to PBX extensions required the services of a local attendant.
In a PBX served by automated attendant services, upon the occurrence of an incoming call from a central office, the incoming trunk circuit is terminated to trip the ringing signal in the central office and a source of a recorded outgoing message (OGM) is connected to the trunk. The message advises the calling party to use the DTMF pad on their telephone to complete the call to a desired extension or to a particular area of the business e.g., the service department. Such systems generally provide access to the PBX attendant by the keying of a specific number, or as a default in the event that a number is not received from the calling party within a specified timeout period of time. This feature is provided because the calling party may have a pulse telephone and cannot provide call signalling to the PBX.
U.S. Pat. No. 4,696,028, which was issued on Sept. 27, 1987, discloses a "PBX INTERCEPT AND CALLER INTERACTIVE BYPASS SYSTEM" which provides automated attendant services as an adjunct to a PBX. Accordingly, the apparatus of this invention requires the presence of an independent PBX. Arrangements such as those shown in the above noted patent, as a basic design concept, rely on centralization for sharing use of facilities. Accordingly, these prior art systems are committed to a relatively large minimum system configuration which is not economical in a residence or in a small business environment.
On-premises intercom systems which connect to a business or residence central office subscriber line, as opposed to a central office trunk, provide certain PBX-like functions in a small business or residence environment; however, the costs associated with the presently available systems, and particularly the cost of the required proprietary station sets and multi-pair station wiring, mitigate against their use.